1. Introduction
This invention relates to polymers and hydrogels of polymers formed from alkyl acrylates or methacrylates, dihydroxyalkyl acrylates or methacrylates, a third comonomer selected from those hereinafter described, and a cross-linking agent. The polymers absorb water to form hydrogels useful in medical and other applications and are especially useful for the formation of contact lenses.
2. Description of the Prior Art
The synthetic aqueous polymer networks known as hydrogels are three dimensional networks of polymers, generally covalently or ionically cross-linked, which interact with aqueous solutions by swelling to some equilibrium state. That is, a hydrogel is a polymeric material which exhibits the ability to swell in water and retain a significant fraction of the imbibed water within its structure. Absent such imbibed water, these materials are properly termed xerogels.
These materials have growing application in the medical field based on their bulk and surface properties. They are especially of interest for their resemblance to living tissue, for example, in their physical properties of high water content and soft, rubbery consistency.
Hydrogel materials are described in U.S. Pat. Nos. 2,976,576 and 3,220,960 incorporated herein by reference. These materials, intended for use as contact lens materials, are hydrogels of a sparingly cross linked hydrophilic copolymer and a substantial quantity of an aqueous liquid, e.g. water. The hydrophilic polymer is a copolymer of a major amount of a polymerizable monoester of an olefinic acid selected from the group of acrylic and methacrylic acids having a single olefinic double bond and a minor amount of a polymerizable diester of one of said acids, the diester having at least two olefinic double bonds. The copolymer is formed by copolymerization in a solvent medium.
Copolymers that are solvent soluble, but water insoluble formed from gylceryl methacrylate and methyl methacrylate are known and described by H. Yasuda, C. E. Lamaza, and L. D. Ikenberry, Makromol. Chem. 118, 1935 (1968) and H. Yasuda, C. E. Lamaza, and A. Peterlin, J. Polym. Sc. Part A-2, 996, 1117-1131 (1971).
In U.S. Pat. No. 4,056,496, incorporated herein by reference, hydrogels are disclosed which are suitable for soft lens fabrication. The hydrogels are formed from a hydrophilic monomer from the group of dihydroxyalkyl acrylates and methacrylates, a substantially water insoluble monomer from the group of alkyl acrylates and methacrylates and preferably a minor amount of an epoxidized alkyl acrylate or methacrylate. The dihydroxyalkyl acrylate is preferably used in major amount, the alkyl acrylate in minor amount, and the epoxidized acrylate in an amount sufficient to impart the desired rigidity. The polymer is formed by a free radical, bulk polymerization process in the substantial absence of solvent. The materials thus formed are useful after hydration as soft contact lens material.
Due to the rapidly expanding areas of application of hydrogel materials, new such materials providing improved and/or different physical properties are greatly sought after. One particular area of improvement is in the fraction of aqueous solution which can be retained by the hydrogel within its structure. Typically, the higher the water content of the hydrogel, the poorer the mechanical properties thereof. Nevertheless, higher water content is often desirable, such as for medical applications where high water content is believed to be a major factor in the biocompatibility of the hydrogel. Of course, water insolubility is essential for most medical application. Accordingly, new materials are sought which will retain a greater fraction of aqueous solution within their structure while having comparable or even improved mechanical properties.